1. Field of the Invention
The present invention relates to downhole drilling and, in particular, to an electrically sequenced tractor (EST) for controlling the motion of a downhole drilling tool in a borehole.
2. Description of the Related Art
The art of drilling vertical, inclined, and horizontal boreholes plays an important role in many industries, such as the petroleum, mining, and communications industries. In the petroleum industry, for example, a typical oil well comprises a vertical borehole which is drilled by a rotary drill bit attached to the end of a drill string. The drill string is typically constructed of a series of connected links of drill pipe which extend between ground surface equipment and the drill bit. A drilling fluid, such as drilling mud, is pumped from the ground surface equipment through an interior flow channel of the drill string to the drill bit. The drilling fluid is used to cool and lubricate the bit, and to remove debris and rock chips from the borehole, which are created by the drilling process. The drilling fluid returns to the surface, carrying the cuttings and debris, through the annular space between the outer surface of the drill pipe and the inner surface of the borehole.
The method described above is commonly termed xe2x80x9crotary drillingxe2x80x9d or xe2x80x9cconventional drilling.xe2x80x9d Rotary drilling often requires drilling numerous boreholes to recover oil, gas, and mineral deposits. For example, drilling for oil usually includes drilling a vertical borehole until the petroleum reservoir is reached, often at great depth. Oil is then pumped from the reservoir to the ground surface. Once the oil is completely recovered from a first reservoir, it is typically necessary to drill a new vertical borehole from the ground surface to recover oil from a second reservoir near the first one. Often a large number of vertical boreholes must be drilled within a small area to recover oil from a plurality of nearby reservoirs. This requires a large investment of time and resources.
In order to recover oil from a plurality of nearby reservoirs without incurring the costs of drilling a large number of vertical boreholes from the surface, it is desirable to drill inclined or horizontal boreholes. In particular, it is desirable to initially drill vertically downward to a predetermined depth, and then to drill at an inclined angle therefrom to reach a desired target location. This allows oil to be recovered from a plurality of nearby underground locations while minimizing drilling. In addition to oil recovery, boreholes with a horizontal component may also be used for a variety of other purposes, such as coal exploration and the construction of pipelines and communications lines.
Two methods of drilling vertical, inclined, and horizontal boreholes are the aforementioned rotary drilling and coiled tubing drilling. In rotary drilling, a rigid drill string, consisting of a series of connected segments of drill pipe, is lowered from the ground surface using surface equipment such as a derrick and draw works. Attached to the lower end of the drill string is a bottom hole assembly, which may comprise a drill bit, drill collars, stabilizers, sensors, and a steering device. In one mode of use, the upper end of the drill string is connected to a rotary table or top drive system located at the ground surface. The top drive system rotates the drill string, the bottom hole assembly, and the drill bit, allowing the rotating drill bit to penetrate into the formation. In a vertically drilled hole, the drill bit is forced into the formation by the weight of the drill string and the bottom hole assembly. The weight on the drill bit can be varied by controlling the amount of support provided by the derrick to the drill string. This allows, for example, drilling into different types of formations and controlling the rate at which the borehole is drilled.
The inclination of the rotary drilled borehole can be gradually altered by using known equipment such as a downhole motor with an adjustable bent housing to create inclined and horizontal boreholes. Downhole motors with bent housings allow the ground surface operator to change drill bit orientation, for example, with pressure pulses from the surface pump. Typical rates of change of inclination of the drill string are relatively small, approximately 3 degrees per 100 feet of borehole depth. Hence, the drill string inclination can change from vertical to horizontal over a vertical distance of about 3000 feet. The ability of the substantially rigid drill string to turn is often too limited to reach desired locations within the earth. In addition, friction of the drilling assembly on the casing or open hole frequently limits the distance that can be achieved with this drilling method.
As mentioned above, another type of drilling is coiled tubing drilling. In coiled tubing drilling, the drill string is a non-rigid, generally compliant tube. The tubing is fed into the borehole by an injector assembly at the ground surface. The coiled tubing drill string can have specially designed drill collars located proximate the drill bit that apply weight to the drill bit to penetrate the formation. The drill string is not rotated. Instead, a downhole motor provides rotation to the bit. Because the coiled tubing is not rotated or not normally used to force the drill bit into the formation, the strength and stiffness of the coiled tubing is typically much less than that of the drill pipe used in comparable rotary drilling. Thus, the thickness of the coiled tubing is generally less than the drill pipe thickness used in rotary drilling, and the coiled tubing generally cannot withstand the same rotational, compression, and tension forces in comparison to the drill pipe used in rotary drilling.
One advantage of coiled tubing drilling over rotary drilling is the potential for greater flexibility of the drilling assembly, to permit sharper turns to more easily reach desired locations within the earth. The capability of a drilling tool to turn from vertical to horizontal depends upon the tool""s flexibility, strength, and the load which the tool is carrying. At higher loads, the tool has less capability to turn, due to friction between the borehole and the drill string and drilling assembly. Furthermore, as the angle of turning increases, it becomes more difficult to deliver weight on the drill bit. At loads of only 2000 pounds or less, existing coiled tubing tools, which are pushed through the hole by the gravity of weights, can turn as much as 90xc2x0 per 100 feet of travel but are typically capable of horizontal travel of only 2500 feet or less. In comparison, at loads up to 3000 pounds, existing rotary drilling tools, whose drill strings are thicker and more rigid than coiled tubing, can only turn as much as 30xc2x0-40xc2x0 per 100 feet of travel and are typically limited to horizontal distances of 5000-6000 feet. Again, such rotary tools are pushed through the hole by the gravity force of weights.
In both rotary and coiled tubing drilling, downhole tractors have been used to apply axial loads to the drill bit, bottom hole assembly, and drill string, and generally to move the entire drilling apparatus into and out of the borehole. The tractor may be designed to be secured between the lower end of the drill string and the upper end of the bottom hole assembly. The tractor may have anchors or grippers adapted to grip the borehole wall just proximal the drill bit. When the anchors are gripping the borehole, hydraulic power from the drilling fluid may be used to axially force the drill bit into the formation. The anchors may advantageously be slidably engaged with the tractor body, so that the drill bit, body, and drill string (collectively, the xe2x80x9cdrilling toolxe2x80x9d) can move axially into the formation while the anchors are gripping the borehole wall. The anchors serve to transmit axial and torsional loads from the tractor body to the borehole wall. One example of a downhole tractor is disclosed in U.S. Pat. No. 6,003,606 to Moore et al. (xe2x80x9cMoore ""606xe2x80x9d). Moore ""606 teaches a highly effective tractor design as compared to existing alternatives.
It is known to have two or more sets of anchors (also referred to herein as xe2x80x9cgrippersxe2x80x9d) on the tractor, so that the tractor can move continuously within the borehole. For example, Moore ""606 discloses a tractor having two grippers. Longitudinal (unless otherwise indicated, the terms xe2x80x9clongitudinalxe2x80x9d and xe2x80x9caxialxe2x80x9d are hereinafter used interchangeably and refer to the longitudinal axis of the tractor body) motion is achieved by powering the drilling tool forward with respect to a first gripper which is actuated (a xe2x80x9cpower strokexe2x80x9d), and simultaneously moving a retracted second gripper forward with respect to the drilling tool (xe2x80x9cresettingxe2x80x9d), for a subsequent power stroke. At the completion of the power stroke, the second gripper is actuated and the first gripper is retracted. Then, the drilling tool is powered forward while the second gripper is actuated, and the retracted first gripper is simultaneously reset for a subsequent power stroke. Thus, each gripper is operated in a cycle of actuation, power stroke, retraction, and reset, resulting in longitudinal motion of the drilling tool.
It has been proposed that the power required for actuating the anchors, axially thrusting the drilling tool, and axially resetting the anchors may be provided by the drilling fluid. For example, in the tractor disclosed by Moore ""606, the grippers comprise inflatable engagement bladders. The Moore tractor uses hydraulic power from the drilling fluid to inflate and radially expand the bladders so that they grip the borehole walls. Hydraulic power is also used to power forward cylindrical pistons residing within propulsion cylinders slidably engaged with the tractor body. Each such cylinder is rigidly secured to a bladder, and each piston is axially fixed with respect to the tractor body. When a bladder is inflated to grip the borehole, drilling fluid is directed to the proximal side of the piston in the cylinder that is secured to the inflated bladder, to power the piston forward with respect to the borehole. The forward hydraulic thrust on the piston results in forward thrust on the entire drilling tool. Further, hydraulic power is also used to reset each cylinder when its associated bladder is deflated, by directing drilling fluid to the distal side of the piston within the cylinder.
Tractors may employ a system of pressure-responsive valves for sequencing the distribution of hydraulic power to the tractor""s anchors, thrust, and reset sections. For example, the Moore ""606 tractor includes a number of pressure-responsive valves which shuttle between their various positions based upon the pressure of the drilling fluid in various locations of the tractor. In one configuration, a valve can be exposed on both sides to different fluid streams. The valve position depends on the relative pressures of the fluid streams. A higher pressure in a first stream exerts a greater force on the valve than a lower pressure in a second stream, forcing the valve to one extreme position. The valve moves to the other extreme position when the pressure in the second stream is greater than the pressure in the first stream. Another type of valve is spring-biased on one side and exposed to fluid on the other, so that the valve will be actuated against the spring only when the fluid pressure exceeds a threshold value. The Moore tractor uses both of these types of pressure-responsive valves.
It has also been proposed to use solenoid-controlled valves in tractors. In one configuration, solenoids electrically trigger the shuttling of the valves from one extreme position to another. Solenoid-controlled valves are not pressure-actuated. Instead, these valves are controlled by electrical signals sent from an electrical control system at the ground surface.
Various types of radially expanding anchors have been used in downhole tractors, such as rigid friction blocks, flexible beams, and engagement bladders. Some advantages of bladders are that they are more radially expandable and thus can operate within certain voids in the earth. Also, bladders can conform to various different geometries of the borehole wall. One known bladder configuration comprises a combination of fiber and rubber. Previous designs utilized Nylon fibers and Nitrile Butadiene Rubber (NBR). The fatigue life of current bladder designs is such that the bladders are able to achieve as much as 7400 cycles of inflation.
One problem with bladders is that they do not resist torque in the tractor body. As the drill bit rotates into the formation, the earth transmits a reactive torque to the bit, which is transmitted proximally through the tractor body. When an engagement bladder is inflated to grip the borehole wall, the compliant bladder tends to permit the tractor body to twist to some degree due to the torque therein. Such rotation can confuse tool direction sensors, requiring an approximation of such reverse twist in the drill direction control algorithm.
Prior art tractors have utilized anchors which permit at least some degree of rotation of the tractor body when the anchor is engaged with an underground borehole wall. A disadvantage of this configuration is that it causes the drill string to absorb reaction torque from the formation. When drilling, the drill bit exerts a drilling torque onto the formation. Simultaneously, the formation exerts an equal and opposite torque to the tractor body. This torque is absorbed partially by the drill string, since the configuration allows rotation of the tractor body when the anchor is actuated. This causes the drill string to twist. If all of the anchors are retracted, which may occur when the tool is to be retrieved, the drill string tends to untwist, which can result in inconsistent advance during walking.
Thus, there is a need for a downhole drilling tractor which overcomes the above-mentioned limitations of the prior art.
Accordingly, it is a principle advantage of the present invention to overcome some or all of these limitations and to provide an improved downhole drilling tractor.
The structural configuration of the tractor, which allows it to work within the harsh environment and limited space within the bore of an oil well, is an important aspect of the invention. An important aspect of the invention is the structural configuration that permits the tractor to fit within an envelope no more than 8.5 inches in diameter and, preferably, no more than 2.875 inches in diameter. This relatively small diameter permits the tractor to work with standard oil well equipment that is designed for 2.875-8.5 inch diameter well bores. Another important aspect of the present invention is the structural configuration that permits the tractor to make relatively sharp turns. Specifically, the tractor desirably has a length of no more than 150 feet, more desirably no more than 100 feet, more desirably no more than 75 feet, more desirably no more than 50 feet, and even more desirably no more than 40 feet. Preferably the length of the tractor is approximately 32 feet. Advantageously, the tractor can turn at least 60xc2x0 per 100 feet of travel. Yet another important aspect of the invention is a structure that permits the tractor to operate at downhole pressures up to 16,000 psi and, preferably, 5,000-10,000 psi, and downhole temperatures up to 300xc2x0 F. and, preferably, 200-250xc2x0 F. Preferably, the tractor can operate at differential pressures of 200-2500 psi, and more preferably within a range of 500-1600 psi (the pressure differential between the inside and outside of the EST, thus across the internal flow channel and the annulus surrounding the tractor).
One limitation of prior art tractors that have valves whose positions control fluid flow providing thrust to the tractor body is that such valves tend to operate only at extreme positions. These valves can be characterized as having distinct positions in which the valve is either on or off, open or closed, etc. As a result, these valves fail to provide fine-tuned control over the position, speed, thrust, and direction of the tractor.
In another aspect, the present invention provides a tractor for moving within a borehole, which is capable of an exceptionally fast response to variations in load exerted on the tractor by the borehole or by external equipment such as a bottom hole assembly or drill string. The tractor comprises a tractor body sized and shaped to move within a borehole, a valve on the tractor body, a motor on the tractor body, and a coupler. The valve is positioned along a flowpath between a source of fluid and a thrust-receiving portion of the body. The valve comprises a fluid port and a flow restrictor. The flow restrictor has a first position in which the restrictor completely blocks fluid flow through the fluid port, a range of second positions in which the restrictor permits a first level of fluid flow through the fluid port, a third position in which the restrictor permits a second level of fluid flow through the fluid port. The second level of fluid flow is greater than the first level of fluid flow. The coupler connects the motor and the flow restrictor, such that movement of the motor causes the restrictor to move between the first position, the range of second positions, and the third position. The restrictor is movable by the motor such that the net thrust received by the thrust receiving portion can be altered by 100 pounds within 0.5 seconds.
One goal of the present invention is to provide a downhole tractor which provides an exceptional level of control over position, speed, thrust, and change of direction of the tractor within a borehole, compared to prior art tractors. Accordingly, in one aspect the present invention provides a tractor for moving within a hole, comprising a tractor body having a plurality of thrust receiving portions, at least one valve on the tractor body, and a plurality of grippers. The valves are positioned along at least one of a plurality of fluid flow paths between a source of fluid and the thrust receiving portions. Each of the plurality of grippers is longitudinally movably engaged with the body and has an actuated position in which the gripper limits movement of the gripper relative to an inner surface of the borehole and a retracted position in which the gripper permits substantially free relative movement of the gripper relative to the inner surface. The plurality of grippers, the plurality of thrust receiving portions, and the valves are configured such the tractor can propel itself at a sustained rate of less than 50 feet per hour and at a sustained rate of greater than 100 feet per hour.
In other embodiments, the tractor can propel itself at sustained rates of less than 30 feet per hour and greater than 100 feet per hour, less than 10 feet per hour and greater than 100 feet per hour, less than 5 feet per hour and greater than 100 feet per hour, less than 50 feet per hour and greater than 250 feet per hour, and less than 50 feet per hour and greater than 500 feet per hour. In another embodiment, the source of fluid has a differential pressure in the range of 200-2500 psi. In another embodiment, the source of fluid has a differential pressure in the range of 500-1600 psi. In another embodiment, the tractor can change the rate at which it propels itself without a change in differential pressure of the fluid. In various embodiments, the tractor has a length preferably less than 150 feet, more preferably less than 100 feet, even more preferably less than 75 feet, even more preferably less than 50 feet, and most preferably less than 40 feet. In various embodiments, the tractor has a maximum diameter preferably less than eight inches, more preferably less than six inches, and even more preferably less than four inches.
In another aspect the present invention provides a tractor comprising a tractor body sized and shaped to move within a borehole, and a valve on the tractor body. The valve is positioned along a fluid flow path between a source of fluid and a thrust-receiving portion of the tractor body, such as a tubular piston. The thrust-receiving portion is sized and configured to receive hydraulic thrust from the fluid.
The configuration of the valve facilitates improved control over the aforementioned properties. In particular, the valve permits precise control over the fluid flowrate along the fluid flow path to the thrust-receiving portion. The valve comprises a valve body and an elongated valve spool. The valve body has an elongated spool passage defining a spool axis, and at least a first fluid port which communicates with the spool passage. The fluid flow path passes through the spool passage and through at least the first fluid port. The valve spool is received within the spool passage and movable along the spool axis. The spool has a flow-restricting segment defining a first chamber within the spool passage on a first end of the flow-restricting segment and a second chamber within the spool passage on a second end of the flow-restricting segment. The flow-restricting segment has an outer radial surface configured to slide along inner walls of the spool passage so as to fluidly seal the first chamber from the second chamber. The flow-restricting segment also has one or more recesses on one of its ends and on its outer radial surface.
The spool has first, second, and third ranges of positions as follows: In the first range of positions, the flow-restricting segment of the spool completely blocks fluid flow through the first fluid port. In the second range of positions, the flow-restricting segment permits fluid flow through the first fluid port only through the recesses. In the third range of positions, the flow-restricting segment permits fluid flow through the first fluid port at least partially outside of the recesses. Advantageously, the flowrate of fluid flowing along the fluid flow path is controllable by controlling the position of the valve spool within the first, second, and third ranges of positions.
In another embodiment, the valve controls the flowrates of fluid to a plurality of different surfaces of the thrust-receiving portion, thereby controlling the net thrust on the tractor body. In yet another embodiment, the tractor body has a second thrust-receiving portion, and a second valve controls the flowrate of fluid flowing thereto.
In another embodiment, the tractor comprises a tractor body, a spool valve, a motor, a coupler, and a gripper. The tractor body has a thrust-receiving portion having a first surface and a second opposing surface. The first surface may be a rear surface, and the second surface may be a front surface. The spool valve comprises a valve body and an elongated spool. The valve body has a spool passage defining a spool axis, and fluid ports which communicate with the spool passage.
Received within the spool passage, the spool is movable along the spool axis to control flowrates along fluid flow paths through the fluid ports and the spool passage. The spool has a first position range in which the valve permits fluid flow from a fluid source to the first surface of the thrust-receiving portion and blocks fluid flow to the second surface. The flowrate of the fluid flow to the first surface varies depending upon the position of the spool within the first position range. The fluid flow to the first surface delivers thrust to the body to propel the body in a first direction in the borehole. The magnitude of the thrust in the first direction depends on the flowrate of the fluid flow (with its associated pressure) to the first surface. The spool also has a second position range in which the valve permits fluid flow from the fluid source to the second surface of the thrust-receiving portion and blocks fluid flow to the first surface. The flowrate of the fluid flow to the second surface varies depending upon the position of the spool within the second position range. The fluid flow to the second surface delivers thrust to the body to propel the body in a second direction in the borehole. The first direction may be downhole, and the second direction may be uphole. The magnitude of the thrust in the second direction depends on the flowrate of the fluid flow to the second surface.
The motor is within the tractor body. The coupler connects the motor and the spool so that operation of the motor causes the spool to move along the spool axis. The gripper is longitudinally movably engaged with the tractor body. The gripper has an actuated position in which the gripper limits movement of the gripper relative to an inner surface of the borehole, and a retracted position in which the gripper permits substantially free relative movement of the gripper relative to the inner surface. Advantageously, the motor is operable to move the spool along the spool axis sufficiently fast to alter the net thrust received by the thrust-receiving portion by 100 pounds within 2 seconds, and preferably within 0.1-0.2 seconds.
In one embodiment, the tractor further comprises one or more sensors and an electronic logic component on the tractor body. The sensors are configured to generate electrical feedback signals which describe one or more of: fluid pressure in the tractor, the position of the tractor body with respect to the gripper, longitudinal load exerted on the tractor body by equipment external to the tractor or by inner walls of the borehole, and the rotational position of an output shaft of the motor. The output shaft controls the position of the spool along the spool axis. The logic component is configured to receive and process the electrical feedback signals, and to transmit electrical command signals to the motor. The motor is configured to be controlled by the electrical command signals. The command signals control the position of the spool.
In another aspect, the present invention provides a tractor having a valve whose, position controls the position, speed, and thrust of the tractor body, and in which fluid pressure resistance to valve motion is minimized. Accordingly, the tractor comprises a body and a valve, motor, coupler, and pressure compensation piston all within the body. The valve is positioned along a fluid flow path from a source of a first fluid to a thrust-receiving portion of the body. The valve is movable generally along a valve axis. The valve has a first position in which the valve completely blocks fluid flow along the flow path, and a second position in which the valve permits fluid flow along the flow path. The coupler connects the motor and the valve so that operation of the motor causes the valve to move along the valve axis. The pressure compensation piston is exposed on a first side to the first fluid and on a second side to a second fluid. The first and second fluids are fluidly separate. The compensation piston is configured to move in response to pressure forces from the first and second fluids so as to effectively equalize the pressure of the first and second fluids. The valve is exposed to the first fluid, and the motor is exposed to the second fluid. Advantageously, the compensation piston acts to minimize the net fluid pressure force acting on the valve along the valve axis, thereby minimizing resistance to valve movement and permitting improved control over the position, speed, thrust, and change of direction of the tractor.
Since the tractor is electric and the motion is controlled electrically, the present invention permits the use of multiple tractors connected in series and simultaneous or non-simultaneous sequencing of the tractors"" packerfeet for various functions. In other words, any number of the tractors can operate simultaneously as a group. Also, some tractors can be deactivated while others are operating. In one example, one tractor can be used for normal drilling with low speeds (0.25-750 feet per hour), and a second tractor in the drill string can be designed for high speeds (750-5000 feet per hour) for faster tripping into the borehole. In another example, two or more tractors can be used with similar performance characteristics. This type of assembly would be useful for applications of pulling long and heavy assemblies into long or deep boreholes. Another example is the use of two or more tractors performing different functions. This type of assembly can have one tractor set up for milling and a second tractor for drilling after the milling job is complete, thus requiring fewer trips to the ground surface. Any combination of different or similar types of tractors is possible.
In another design variation, the tractor can be formed from less expensive materials, such as steel, resulting in decreased performance capability of the tractor. Such a low cost tractor can be used for specialized applications, such as pulling specialty oil production apparatus into the borehole and then leaving it in the hole. Sliding sleeve sand filter production casing can be installed in this manner.
Another goal of the present invention is to provide a downhole tractor for drilling or moving within a borehole, which is capable of turning at significantly high angles while pulling or pushing a large load and/or while minimizing twisting of the tractor body. Accordingly, in another aspect the present invention provides a tractor for moving within a borehole, comprising an elongated body, a gripper, and a propulsion system on the body. The body is configured to push or pull equipment within the borehole, the equipment exerting a longitudinal load on the body. The gripper is longitudinally movably engaged with the body. The gripper has an actuated position in which the gripper limits movement between the gripper and an inner surface of the borehole, and a retracted position in which the gripper permits substantially free relative movement between the gripper and the inner surface. The propulsion system is configured to propel the body through the borehole while the gripper is in its actuated position.
Advantageously, the body is sufficiently flexible such that the tractor can preferably turn up to 300, more preferably 45xc2x0, and even more preferably 60xc2x0 per 100 feet of travel, while pushing or pulling a longitudinal load. The particular load which the body can push or pull while exhibiting this turning capability depends upon the body diameter. Various embodiments of the invention include tractors having diameters of 2.175 inches, 3.375 inches, 4.75 inches, and 6.0 inches. Note that other embodiments are also conceived. A tractor having a diameter of 2.175 inches desirably has the above-mentioned turning capability while pushing or pulling loads up to 1000 pounds, and more desirably up to 2000 pounds. The same information for other embodiments is summarized in the following table:
It should be noted that as the maximum diameter of the tractor""s pistons, shafts, and control assembly increase so also shall the maximum thrust-pull and speed. These and other design considerations can be adjusted for optimum performance with respect to maximum and minimum speed, maximum and minimum pull-thrust, control response times, turning radius, and other desirable performance characteristics.
In one embodiment, the tractor has large diameter segments and small diameter segments. The large diameter segments include one or more of (1) a valve housing having valves configured to control the flow of fluid to components of the propulsion system, (2) a motor housing having motors configured to control the valves, (3) an electronics housing having logic componentry configured to control the motors, (4) one or more propulsion chambers configured to receive fluid to propel the body, (5) pistons axially movable within the propulsion chambers, and (6) the gripper. For the tractor having a diameter of 3.375 inches, the large diameter segments have a diameter of at least 3.125 inches. The small diameter segments have a diameter of 2.05 inches or less and a modulus of elasticity of 19,000,000 or more. Substantially all of the bending of the tractor occurs in the small diameter segments.
In another aspect, the present invention provides a tractor for moving within a borehole, comprising an elongated body, at least a first gripper, and a propulsion system on the body. The body defines a longitudinal axis and is configured to transmit torque through the body. In particular, the body is configured so that when the body is subjected to a torque about the longitudinal axis below a certain value, twisting of the body is limited to no more than 5xc2x0 per movement of a gripper, i.e., per on stroke length of a propulsion cylinder. These values vary depending upon the tractor diameter, and are summarized in the table below:
The first gripper is axially movably engaged with the body. The first gripper has an actuated position in which the first gripper limits movement of the first gripper relative to an inner surface of the borehole, and a retracted position in which the first gripper permits substantially free relative movement between the first gripper and the inner surface. The first gripper is rotationally fixed with respect to the body so that the first gripper resists rotation of the body with respect to the borehole when the first gripper is in the actuated position. A second gripper may also be provided, which is configured identically to the first gripper and is also axially movably engaged with the body. The propulsion system is configured to propel the body when at least one of the grippers is in its actuated position. Advantageously, the body is sufficiently flexible such that the tractor can turn up to 60xc2x0 per 100 feet of longitudinal travel.
Another goal of the present invention is to provide an improved gripper for a downhole tractor used for moving within a borehole. Accordingly, in yet another aspect the invention provides a tractor for moving within a borehole, comprising an elongated body and a packerfoot configured to provide enhanced radial expansion compared to the prior art. The packerfoot comprises an elongated mandrel longitudinally movably engaged on the body, and a generally tubular bladder assembly concentrically engaged on the mandrel. The bladder assembly comprises a generally tubular inflatable bladder having a radial exterior, a first mandrel engagement member attached to a first end of the bladder and engaged with the mandrel, a second mandrel engagement member attached to a second end of the bladder and engaged with the mandrel, a plurality of longitudinally oriented flexible beams on the radial exterior of the bladder, a first band securing the first ends of the beams against the first mandrel engagement member, and a second band securing the second ends of the beams against the second mandrel engagement member. The beams have first ends at the first end of the bladder and second ends at the second end of the bladder. The beams are configured to flex and grip onto a borehole when the bladder is inflated.
In one embodiment, the mandrel is non-rotatably engaged on the body. In another embodiment, the first mandrel engagement member is fixed to the mandrel, the second mandrel engagement member is longitudinally slidably engaged with the mandrel, and the second tube portion is non-rotatable with respect to the mandrel. In another embodiment, the tractor of the present invention can be fitted with different sizes of packerfeet, which allows the tractor to enter and operate in a range of hole sizes.
In another aspect, the present invention provides a downhole tractor having a xe2x80x9cflextoe packerfoot,xe2x80x9d in which separate components provide outward radial force for gripping a borehole and torque transmission from the tractor body to the borehole. Accordingly, a tractor for moving within a borehole comprises an elongated body, an elongated mandrel longitudinally movably engaged with the body, and a gripper assembly. The gripper assembly comprises one or more inflatable bladders on the mandrel, and one or more elongated beams. The beams have first ends fixed to the mandrel on a first end of the bladder, and second ends longitudinally movably engaged with the mandrel on a second end of the bladder. The bladder has an inflated position in which the bladder or the beams limit movement of the gripper assembly relative to an inner surface of the borehole, and a deflated position in which the bladder or the beams permit substantially free relative movement between the gripper assembly and the inner surface. The beams are configured to flex radially outward to grip the inner surface of the borehole when the bladder is in the inflated position. The beams are also configured to transmit torque from within the body to the inner surface of the borehole.
In one embodiment, the bladder is configured to apply a radially outward force onto the beams when the bladder is in the inflated position, which causes the beams to flex outward and grip the inner surface of the borehole. In another embodiment, the mandrel is non-rotatably engaged with the body so that the body is prevented from rotating with respect to the inner surface of the borehole when the bladder is in the inflated position. In another embodiment, the first ends of the beams are hingedly secured to the mandrel, and the second ends of said beams are hingedly secured to a shuttle configured to slide longitudinally on the mandrel. The shuttle is non-rotatable with respect to the mandrel.
Another goal of the present invention is to provide a downhole tractor having an improved, longer-lasting inflatable bladder for gripping onto the inner surface of a borehole. In particular, the bladder has a higher fatigue life than prior art bladders. Accordingly, the present invention provides a tractor for moving within a borehole, comprising an elongated body defining a longitudinal axis, and an inflatable bladder longitudinally movably engaged with the body. The bladder is formed from an elastomeric material reinforced by fibers oriented in two general directions crossing one another at an angle of between 0xc2x0 and 90xc2x0 woven together, more preferably between 14xc2x0 and 60xc2x0, and even more preferably between approximately 30xc2x0 and 40xc2x0. The bladder has an inflated position in which the bladder limits movement of the bladder relative to an inner surface of the borehole, and a deflated position in which the bladder permits substantially free relative movement between the bladder and the inner surface.
The above-described embodiments of the invention, which utilize the drilling fluid to provide power for the tool, have specific design considerations to optimize tool operational life. Experiments have shown that drilling fluids can rapidly erode many metals, including Stabaloy and Copper-Beryllium if drilling fluid velocities within the tool are exceeded. It is another aspect of this invention to limit fluid velocities on straight sections within the tool to less than 35 feet per second, unless high abrasion resistant materials are used or other geometrical flow path considerations are used. It is known that at higher velocities erosion occurs within the tool, which limits the operational life of tractor components. Operational life is significant in that downhole failures and tool retrievals are extremely costly.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described herein above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.